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The centers of our galaxy and the nearby Messier 87 are known to containsupermassive black holes, which support accretion flows that radiate across theelectromagnetic spectrum. Although the composition of the accreting gas isunknown, it is likely a mix of ionized hydrogen and helium. We use a simpleanalytic model and a suite of numerical general relativisticmagnetohydrodynamic accretion simulations to study how polarimetric images andspectral energy distributions of the source are influenced by thehydrogen/helium content of the accreting matter. We aim to identify generaltrends rather than make quantitatively precise predictions, since it is notpossible to fully explore the parameter space of accretion models. If theion-to-electron temperature ratio is fixed, then increasing the helium fractionincreases the gas temperature; to match the observational flux densityconstraints, the number density of electrons and magnetic field strengths musttherefore decrease. In our numerical simulations, emission shifts from regionsof low to high plasma beta -- both altering the morphology of the image anddecreasing the variability of the light curve -- especially in stronglymagnetized models with emission close to the midplane. In polarized images, wefind that the model gas composition influences the degree to which linearpolarization is (de)scrambled and therefore affects estimates for the resolvedlinear polarization fraction. We also find that the spectra ofhelium-composition flows peak at higher frequencies and exhibit higherluminosities. We conclude that gas composition may play an important role inpredictive models for black hole accretion.

Effects of Hydrogen vs. Helium on Electromagnetic Black Hole Observables